For every normal submission that ends up being a return, you'll always get some kind of short description regarding why it was returned. Custom critiques are simply a more in-depth version of that, and I believe you are even allowed to ask something specific regarding your song as you submit it.

I've personally never done it, but I've heard good things from those that have. Not necessary for everybody, but it sounds like it can be a great tool in the right circumstances.

Further to Andrew's remark, I have done it in the past and it definitely is a good idea to formulate questions (in the lyrics field). Given you choose what genre your music is in, you will (I assume) get an expert in that area who can tell you whether your track is up to standard and, if not, give you an idea why not. But I think you should not expect answers to be very specific in all fields. For instance, if you are more concerned about how good the technical production is, you might not necessarily get an expert in that field who will e.g. tell you to EQ more in the 800Hz-1000Hz range or similar. The screener - so I assume - will give you information that she/he feels confident about given his/her specific background.